1. Field of the Invention
The present invention relates to a hot-melt type ink composition for use in an ink jet printing apparatus, and more particularly to such a hot-melt type ink composition that is to be fused by heating at an elevated temperature above room temperature, and discharged in a liquid phase from the ink jet apparatus, so as to print images on a recording medium.
2. Discussion of Related Art
There have been proposed various ink jet printing methods, which include: electrostatic ink jet printing which utilizes electrostatic force for discharging or ejecting an ink; drop on demand type ink jet printing which utilizes displacement of piezoelectric elements which are energized in response to the application of a pulsed voltage, so as to produce pressure for ejecting the ink; thermal ink jet printing in which the ink is heated to a high temperature to form and develop bubbles, so as to produce pressure for ejecting the ink. These known ink printing methods permit printing of images with a high resolution and good print quality.
Generally, a water-based ink containing water as a major solvent, or an oil-based ink containing an organic solvent is used for the above types of ink jet printing. Print images obtained by the water-based ink generally have poor water-fastness, while print images obtained by the oil-based ink have excellent water-fastness.
However, the known water-based and oil-based inks are usually in a liquid phase at room temperature, and are therefore likely to suffer from blurring of images printed on a recording medium, and insufficient print density. Further, some component of the ink may be deposited or precipitated due to the liquid phase of the ink, resulting in undesirably lowered reliability of the ink jet printing.
In view of the above problems, it is proposed to use a hot-melt type oil-based ink for ink jet printing, which ink is in a solid phase at room temperature. For example, U.S. Pat. No. 3,653,932 discloses an ink composition containing dialkylester sebacate, and U.S. Pat. No. 4,390,369 (corresponding to JP-A-58-108271) discloses an ink composition containing at least one natural wax, while U.S. Pat. No. 4,758,276 (corresponding to JP-A-59-22973) discloses an ink composition containing stearic acid. Further, U.S. Pat. No. 4,822,418 (corresponding to JP-A-61-83268) discloses an ink composition containing an acid or alcohol having from 20 to 40 carbon atoms, and ketone having a relatively high melting point, and U.S. Pat. No. 4,820,346(corresponding to JP-A-62-48774) discloses an ink containing a thermosetting resin having a high hydroxyl number, a solid organic solvent having a melting point less than 150.degree. C., and a small amount of a dye. U.S. Pat. No. 4,870,118 (corresponding to JP-A-62-112627) discloses an ink having a coloring material, a first solvent which is in a solid phase at room temperature and is liquefied or fused by heating at an elevated temperature above the room temperature, and a second solvent for dissolving the first solvent, which is in a liquid phase at room temperature and has a high volatility. JP-A-62-295973 discloses an ink containing a synthetic wax having polar groups with a dye soluble therein.
on the other hand, JP-A-2-29471 discloses an ink in which a dye is easily dissolved by use of C.sub.1 to C.sub.22 fatty acid amide, while JP-A-5-125316 discloses an ink including a dye-dissolving agent, such as fatty acid or fatty acid amide, having a melting point of 80.degree. C. or higher.
However, the hot-melt inks disclosed in the above publications are not able to meet some basic requirements, such as high clearness or sharpness of colors of images, and high adhesive property of the ink with respect to a recording medium, for example. These hot-melt inks are thus unsatisfactory in the print quality of images obtained therefrom. In particular, the inks are lack of transparency, and thus cannot be used for overhead projectors (OHP).
Further, vehicles of the known hot-melt inks as described above do not have sufficiently high capability of dissolving dyes, that is, cannot dissolve an amount of dye(s) required for providing sharp color images on a paper. The low dissolving capability of the vehicles makes it difficult to produce inks having desired colors. Generally, dyes for cyan blue and black colors have low solubility. Thus, it is difficult to produce inks capable of providing sharp cyan blue and black images.